In the application field of ultra-high precision optical lens such as lithographic projection lens, the requirement of lens performance is high and tolerance is tight. For an optical system assembled with a number of lenses, an aberration due to accumulated deformation of multiple lenses would lead to degeneration of optical quality. On the other hand, the optical quality of a large size lens will degenerate with the surface deformation produced by the self-gravity of the lens and the clamping stress of its retainer. Therefore, the retainers of the lenses need to be specially designed to maintain the optical quality.
The lithographic projection lens in the semiconductor industry projects the pattern of a mask onto a wafer. The required linewidth resolution of the pattern on wafer is at least micro-scale or finer nano-scale, and so the lithographic projection lens is a diffraction limit optical system and the optical quality is evaluated using wavefront error. The wavefront will be changed and distorted by the lens power and surface error, such as the surface form accuracy or surface deformation. When the light passing through all the lens elements and the wavefront is distorted, the total wavefront error is the superposition of each distorted wavefront of lens. Therefore, even though the lenses are perfect and located at the correct positions, the assembly of large size lenses (e.g., with diameter ≥100 mm) would still have the problems of the above-mentioned lens surface deformation and stress caused by the self-gravity of the lens and the clamping of the retainer, resulting in degeneration in the optical performance, and thus good image quality is not guaranteed.
Most of the lithographic projection lenses are vertical setups. In the prior art, the three-point supporting retainer technique supplemented by an elastic supporting structure retains the lens in a lens barrel, to reduce the clamping stress and lens deformation. However, even if there are special designs on the retainers, the residual surface deformation would still exist. A lithographic projection lens is usually composed of ten or more lenses, the wavefront error from the accumulation and superimposition of the residual wavefront distortions caused by the deformed lenses reduces the final optical quality.
Lenses are rotated about their lens axes, or polarization elements are added to compensate for aberration in the prior art. Such techniques are common in the production of lithographic projection lenses. For example, lithographic lens supplier Carl Zeiss SMT and lithography system manufacturer ASML have both disclosed the above techniques. But their features and problems to be solved are different from those of the present invention.
Carl Zeiss SMT discloses in U.S. Pat. No. 6,697,199 B2 “Objective with lenses made of a crystalline material” and U.S. Pat. No. 7,239,447 B2 “Objective with crystal lenses” that through rotating lenses about their lens axes, the phase retardation produced by the intrinsic birefringence of optical materials with cubic lattice structures is compensated, to reduce the effect of wavefront error on the optical quality.
ASML discloses in U.S. Pat. No. 6,970,232 B2 “Structures and methods for reducing aberration in integrated circuit fabrication systems” and U.S. Pat. No. 7,738,172 B2 “Structures and methods for reducing aberration in optical systems” that by utilizing polarization optical elements configured inside a lithographic projection lens to rotate the polarization of light about an axis, the phase retardation produced by the intrinsic birefringence of optical materials with cubic lattice structures is compensated. In addition, compensation effect can also be achieved through configuring different optical material lenses with cubic lattice structures to rotate the polarization of light about an axis.
James P. McGuire discloses methods of aberration compensation in optical systems in US patent publication No. 2003/0086171 A1 “Methods for reducing aberration in optical systems.” The patent proposes a solution for the aberration in optical lens caused by polarizations and cubic optical materials, and utilizes the configuration of the polarization states among the polarization modulating elements in a first group, a second group and between the first and the second groups in the lens to compensate for the aberration.
The prior art mostly discloses methods of aberration compensation for high numerical aperture lithographic projection lens containing optical material with a cubic lattice structure such as CaF2, where the aberration or the wavefront error come from the intrinsic birefringence produced by the lattice structure of the cubic optical material itself. The methods disclosed in the prior art frequently have the disadvantages of materials being difficult to manufacture, high cost or the lens being difficult to mass produce, and they do not compensate for the aberration caused by the self-gravity of the lens and the clamping of its retainer.
In order to overcome the drawbacks in the prior art, a lens assembly device and an adjustment method for lenses in a lens assembly are disclosed.